This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. Section 119 from an application for xe2x80x9cReference Wavelength Providing Device for Performance Monitor in WDM Optical Transmission Systemxe2x80x9d filed in the Korean Industrial Property Office on May 12, 1999 and there duly assigned Serial No. 99-17015.
1. Field of the Invention
The present invention relates generally to a performance monitoring device in an optical transmission system using wavelength division multiplexing (WDM) technologies, and in particular, to a device for providing a reference wavelength that can be used in the performance monitoring device.
2. Description of the Related Art
FIG. 1 illustrates a schematic view of a basic WDM optical transmission system. As shown in FIG. 1, the WDM optical transmission system is provided with a transmitter 100, a transmission line unit 106, and a receiver 110. The transmitter 100 includes a plurality of channel transmitters from 1021 to 102N and a wavelength multiplexer (MUX) 104. The channel transmitters 1021 to 102N are coupled to the SDH (Synchronous Digital Hierarchy) equipment and the ATM (Asynchronous Transfer Mode) equipment that are supported by a particular WDM transmission equipment or exchange equipment for receiving optical data therefrom. Each of the transmitters modulates an optical signal received therein at an assigned wavelength through the optical/electrical/optical conversion and feeds the converted optical signal to the wavelength MUX 104. The wavelength MUX 104 multiplexes the modulated data at different wavelengths received from the transmitters 1021 to 102N and transmits the multiplexed optical signal to the remote receiver 110 through a transmission line unit 106.
Generally, in the transmission of optical signals at 2.5 Gbps, an optical amplifier is required for the transmission distance higher than 80 km. Thus, a plurality of optical amplifiers 1081 to 108N is installed in sequence along the transmission line unit 106. A series of EDFAs (Erbium-doped Fiber Amplifiers) are used as the optical amplifiers 1081 to 108N to serve as power amplifiers, line amplifiers, and preamplifiers. The receiver 110, which includes a wavelength demultiplexer (DEMUX) 112 and a plurality of channel receivers 1141 to 114N, receives a composite light beam having component light beams of different wavelengths originated from the transmitter 100 through the transmission line unit 106. The wavelength DEMUX 112 separates the composite input optical signal into component light beams of different wavelengths respectively and feeds the separated light beam to the channel receivers 1141 to 114N. Then, the channel receivers 1141 to 114N subject the received optical signal to the opto-electric conversion for further processing.
When optical signals are multiplexed, fluctuations in the wavelength of the transmitter and the wavelength characteristics of the optical wavelength multiplexer/demultiplexer result in the deterioration of receiver sensitivity. Thus, monitoring the wavelength throughout the system is an essential technique. As the spacing between the wavelengths of the WDM signal light are close to each other, it is important to maintain each signal at its wavelength to avoid crosstalk between channels. To monitor the spacing between the channels, various performance monitors are used in the transmitter, receiver, and optical amplifiers. WDM systems have rapidly developed moving to 8 and 16 (and higher) wavelength systems with the channel transmission rate of 2.5 Gbps and a transmission capacity of 20 or 40 Gbps. Thus, any possible malfunction of the component devices can deteriorate the multitude of communications services in the WDM optical transmission system. This is the reason why it is so important and necessary to monitor the various characteristics of the WDM signal light in future optical wave networks.
For example, it may be necessary to monitor (a) the total input/output optical power, (b) an optical power in each input/output channel, (c) an Optical Signal-to-Noise Ratio (OSNR) of each channel, and (d) a wavelength in each channel. The total input/output power is easily monitored through the implementation of an optical tap coupler and a photo-diode. However, to monitor the optical output, the OSNR, and the wavelength in each channel of WDM signal light, each multiplexed optical signal should be separated into component light beams of different wavelengths respectively. Hence, it is expensive to implement such a monitoring device.
The wavelength of optical signal, the OSNR, and the input light intensity of an optical amplifier are important characteristics to monitor for high accuracy. A slight variation in these values can affect the output of an optical amplifier and accelerate the gain tilt and cause a great output difference in the wavelengths.
Currently, different companies in various countries have deployed their own performance monitoring devices for monitoring the WDM optical transmission systemxe2x80x94such as the LG and the KAIST in Korea; the Lucent, the NTT, the Queensgate, and the HP in other countries. These various performance monitoring technologies can be categorized into three types.
(1) The optical signal is demultiplexed using an optical demultiplexer for monitoring each output wavelength. In this area, the Lucent uses fiber gratings as the demultiplexer, the HP uses AWGs (Arrayed Waveguide Gratings) as the demultiplexer, and the LG uses a coupler and a fixed filter as the demultiplexer.
This first type of technologies has shortcomings in that the fiber grating, by the Lucent, still needs to improve its long-term stability and reliability. The AWGs, by the HP, are too expensive, and the use of a coupler and a fixed filter, used the LG, increases the size and cost of the monitoring device. In a further attempt to accurately detect an accurate wavelength, the HP implemented a device for providing a reference wavelength using an Hexe2x80x94Ne laser on the market. However, the Hexe2x80x94Ne laser, which is a gas laser, is not suitable for the transmission equipment since it has to be replaced whenever the pressure of the filled gas drops below a certain level.
(2) A pilot tone is used by the Lucent and the KAIST to monitor various characteristics of the WDM signal light. An optical transmitter operating at 2.5 Gbps or higher transmits an additional modulation signal in a voice frequency band. Then, the optical output, the OSNR, and the wavelengths at each wavelength are measured using the modulated pilot tone. The problem with the second technology is that the introduction of pilot tone signals along the transmission path can create noises at the receiving end, thus hindering the measurement of each wavelength with high accuracy.
(3) A variable optical filter is used to monitor the WDM signal light, which was disclosed by the NTT at the Optical Fiber Communication Conference (OFC) in 1998. The variable optical filter is implemented in conjunction with the sawtooth waveforms, so as to detect the variations in the optical outputs with respect to wavelengths changed with the passage of time passage are measured. Then, the optical output and the OSNR at each wavelength are measured.
FIG. 2 depicts the performance monitor 204 configured according to the third technology. As illustrated in FIG. 2, an input optical signal is applied to a tap coupler 200. The tap coupler 200 extracts a portion of the optical signal so that the extraction has no influence on the transmission signal. A variable optical filter 206 filters the output of the tap coupler 200 at wavelengths limited by the sawtooth waves received from a sawtooth wave generator 208. Accordingly, transmission occurs only where the transmission conditions for both filters line up. Thus, for the input of the sawtooth wave, as shown in (a) of FIG. 3, the filtering wavelength is varied according to the voltage of the sawtooth wave, then the optical transmission signals are outputted at each wavelength xcex1, xcex2, xcex3, xcex4, . . . xcexn, as shown in (b) of FIG. 3. A photo-diode 210 converts the optical transmission data received from the variable optical filter to an electrical signal. A processor 212 measures the optical power and the OSNR of each channel from the electrical signal received from the photo-diode 210.
However, this third method is still inferior in measuring the wavelengths with high accuracy. To solve this problem, the KAIST has suggested a method of generating a reference wavelength using an optical grating filter and then measuring the accurate optical output, the OSNR, and the wavelength of each channel through the implementation of a variable optical filter. Normally, the optical signals are extracted from the output/input of an EDFA but recently they are also extracted from a port. This method also has problem in that a device to be monitored, like the EDFA, has to generate light at the reference wavelength causing noises and the light signal has to be extracted from two locations.
Moreover, wavelengths can not be determined accurately because the wavelength of AWG or fiber gratings used in the prior art are influences by ambient temperature. Thus, even if the performance monitor in the prior art provides the output power and the OSNR at each wavelength, there is no guarantee that they are accurate.
Furthermore, a gas laser and an optical fiber grating filter have been used to provide a reference wavelength but they are expensive, and at least two tap points are needed.
It is, therefore, an object of the present invention to provide a reference wavelength providing a device for a performance monitor in a WDM optical transmission system, which can provide a reference wavelength economically by incorporating a simple design.
The above object of the present invention can be achieved by providing a reference wavelength providing device for a performance monitor in a WDM optical transmission system. The device includes: a light emitting diode (LED) generating light in a predetermined wavelength band; a filter receiving light from the LED and filtering only an optical signal at a predetermined reference wavelength; a tap coupler extracting a part of the optical transmission data; a coupler coupling the output of the tap coupler with the output of the filter; a sawtooth wave generator generating sawtooth waves in a voltage range corresponding to the range that includes the reference wavelength and the wavelengths of a plurality of channels of the optical transmission signal; and, a variable optical filter receiving the output of the coupler and the sawtooth waves and filtering the output of the coupler while varying the filtering wavelength according to the voltages of the sawtooth waves.